Kalyan Bhanja

Effects of Concentration on Like-Charge Pairing of Guanidinium Ions and on the Structure of Water: An All-Atom Molecular Dynamics Simulation Study

Like-charge ion-pair formation in an aqueous solution of guanidinium chloride (GdmCl) has two important facets. On one hand, it describes the role of the arginine (ARG) side chain in aggregation and dimer formation in proteins, and on the other hand, it lends support for the direct mechanism of protein denaturation by GdmCl. We employ all-atom molecular dynamics simulations to investigate the effect of GdmCl concentration on the like-charge ion-pair formation of guanidinium ions (Gdm(+)). From analyses of the radial distribution function (RDF) between the carbon atoms of two guanidinium moieties, the existence of both contact pairs and solvent-separated pairs has been observed. Although the peak height corresponding to the contact-pair state decreases, the number of Gdm(+) ions in the contact-pair state actually increases with increasing GdmCl concentration. We have also investigated the effect of the concentration of Gdm(+) on the structure of water. The effect of GdmCl concentration on the radial and tetrahedral structures of water is found to be negligibly small; however, GdmCl concentration has a considerable effect on the hydrogen-bonding structure of water. It is demonstrated that the presence of chloride ions, not Gdm(+), in the first solvation shell of water causes the distortion in the hydrogen-bonding network of water. In order to establish that Gdm(+) not only stacks against another Gdm(+) but also directly attacks the ARG residue of a protein or peptide, simulation of an ARG-rich peptide in 6 M aqueous solution of GdmCl has been performed. The analyses of RDFs and orientation distributions reveal that the Gdm(+) moiety of the GdmCl attacks the same moiety in the ARG side chain with a parallel stacking orientation.

Investigation of holdup and axial dispersion of liquid phase in a catalytic exchange column using radiotracer technique

Holdup and axial dispersion of liquid phase in a catalytic exchange column were investigated by measuring residence time distributions (RTD) using a radiotracer technique. RTD experiments were independently carried out with two different types of packings i.e. hydrophobic water-repellent supported platinum catalyst and a mixture (50% (v/v)) of hydrophobic catalyst and a hydrophillic wettable packing were used in the column. Mean residence times and hold-ups of the liquid phase were estimated at different operating conditions. Axial dispersion model (ADM) and axial dispersion with exchange model (ADEM) were used to simulate the measured RTD data. Both the models were found equally suitable to describe the measured data. The degree of axial mixing was estimated in terms of Peclet number (Pe) and Bodenstein number (Bo). Based on the obtained parameters of the ADM, correlations for total liquid hold-up (HT) and axial mixing in terms of Bo were proposed for design and scale up of the full-scale catalytic exchange column.

Estimation of Tritium Release from LLCB TBM and Its Ancillary Systems and Tritium Management in Different Locations of ITER

Abstract The Indian Lead Lithium Ceramic Breeder (LLCB) Test Blanket Module (TBM) is to be installed in one half of equatorial port #2 for testing in ITER machine. Liquid Pb-Li and solid Li2TiO3 are the tritium breeder materials in LLCB TBM. Tritium permeates through structural materials in particular at higher temperatures, which is a major operational and safety concern. Therefore, tritium flows in different locations of ITER Tokamak complex have been estimated. Tritium transport from LLCB TBM and its ancillary systems into process rooms has been studied and analyzed in this work. A steady state diffusion limited permeation model neglecting surface effects has been used for the analysis. Tritium permeation to the Vacuum Vessel, Pipe Forest Area, Port Cell, Pipe Chase Area, Tokamak Cooling Water System Vault Annex (TCWS-VA) and Tritium Process Room in L-2 level has been estimated. The requirement to be fulfilled in each equatorial port cell is that the tritium concentration in the port cell during maintenance operations should be below the admissible limit for human access (regulatory maximum allowable value < 1 DAC = 3.4 × 105 Bq/m3, Derived Air concentration). The presence of the Detritiation System (DS) in the Port cell has to be taken into account.This admissible limit for human access has to be reached in a sufficiently short time (target = 12 h) after plasma shutdown. Additional release during maintenance and radiological zoning with recommended <10 µSv/h need to be considered. Management of concentration of permeated tritium in different locations considering above requirement has also been discussed in this paper.